Spalted apple is technically not a specific species of apple. Spalting is any form of wood coloration caused by fungi and can also occur even in living trees.
It compares in density with cherry, a bit softer than maple, beautiful color, and is stiff and musical when tapped. It is more in the warm type of tonewoods like mahogany or walnut, but it also seems to have clarity and sustain in the high register.